I think it would be a good idea if threads that have been inactive (i.e. haven't been replied to) for a certain period of time (e.g. 3 months) would be closed automatically. Not only would this stop old threads suddenly re-appearing on the front page, but it would also allow these older threads to slip down the forum and be replaced by more recent ones.
Of course, this shouldn't affect sticky/announcement threads (because they need to be active all the time), and older threads would still be available for people to read, it's just that they wouldn't be able to post to them.